588 DEVELOPMENT OF THE IMPREGNATED EGG. 



against the internal surface of the vitelline membrane. (Fig 194, d.) 

 They have by this time been converted into true animal cells; and 

 these cells, adhering to each other by their adjacent edges, form a 

 continuous organized membrane, which is termed the Blastodermic 

 membrane. 



During the formation of this membrane, moreover, the egg, while 

 passing through the Fallopian tubes into the uterus, has increased 

 in size. The albuminous matter with which it was enveloped has 

 liquefied ; and, being absorbed by endosmosis through the vitelline 

 membrane, has furnished the materials for the more solid and ex- 

 tensive growth of the newly-formed structures. It may also be 

 seen that a large quantity of this fluid has accumulated in the 

 central cavity of the egg, inclosed accordingly by the blastodermic 

 membrane, with the original vitelline membrane still forming an 

 external envelope round the whole. 



The next change which takes place consists in the division or 

 splitting of the blastodermic membrane into two layers, which are 

 known as the external and internal layers of the blastodermic membrane. 

 They are both still composed exclusively of cells ; but those of the 

 external layer are usually smaller and more compact, while those 

 of the internal are rather larger and looser in texture. The egg 

 then presents the appearance of a globular sac, the walls of which 

 consist of three concentric layers, lying in contact with and inclos- 

 ing each other, viz., 1st, the structureless vitelline membrane on the 

 outside ; 2d, the external layer of the blastodermic membrane, com- 

 posed of cells ; and 3d, the internal layer of the blastodermic mem- 

 brane, also -composed of cells. The cavity of the egg is occupied 

 by a transparent fluid, as above mentioned. 



This entire process of the segmentation of the vitellus and the 

 formation of the blastodermic membrane is one of the most re- 

 markable and important of all the changes which take place during 

 the development of the egg. It is by this process that the simple 

 globular mass of the vitellus, composed of an albuminous matter 

 and oily granules, is converted into an organized structure. For 

 the blastodermic membrane, though consisting only of cells nearly 

 uniform in size and shape, is nevertheless a truly organized mem- 

 brane, made up of fully formed anatomical elements. It is, more- 

 over, the first sign of distinct organization which makes its appear- 

 ance in the egg ; and as soon as it is completed, the body of the 

 new foetus is formed. The blastodermic membrane is, in fact, the 

 body of the foetus. It is at this time, it is true, exceedingly simple 



